In recent years, using anionic polymerization technique to synthesize star polymers is increasingly stressed, its research scope is continuously enlarged, the said polymer species are growing day after day and a lot of which have embodied industrial production. The field involved was started from the star thermoplastic elastomer coupled star block copolymer of styrene-butadiene ((SB).sub.n R) and coupled star block copolymer of styrene-isoprene ((SI).sub.m R) at the very beginning, and developed now into coupled star random solution polymerized polystyrene-butadiene (S-SBR), low cis polybutadiene (LCBR), medium vinyl polybutadiene (MVBR), and high vinyl polybutadiene (HVBR), even into the latest styrene-isoprene-butadiene rubber (S-SIBR) etc.
There are generally two kinds of processes for anionically synthesizing star polymers. One is using multifunctional organic alkali metal initiator for the synthesis, especially the multifunctional organic lithium initiator; but up to now, the anionic process for synthesizing star polymers is usually a synthesis using a monofunctional alkyllithium as the initiator, firstly synthesizing linear polymer and then adding multifunctional coupling agent for carrying out the coupling reaction to obtain the final product.
Organic lithium initiator is widely used in the anionic polymerization, said organic lithium initiator may be divided into several sorts depending on the number of active centers each molecule contains after being initiated, i.e., monolithium initiator such as RLi; dilithium initiator such as naphthyllithium; and multifunctional organic lithium initiator (wherein the number of active centers each molecule contains is greater than two).
Multifunctional organic lithium initiator is principally used in star polymer synthesis. In comparison with linear high polymers, the star polymers have their own features, for example, in comparison with linear solution polymerized styrene-butadiene rubber, the star solution polymerized styrene-butadiene rubber has better overall mechanical performances after tire-making, mainly a lower rolling resistance, a better wet skid resistance, it is a kind of energy-saving rubber; for further example, in comparison with the linear products, the star medium, vinyl polybutadiene has significantly better processing performance and cold flow resistance; further, with respect to thermoplastic elastomer, the star SBS has, in comparison with linear SBS, higher Mooney viscosity, better tensile strength and higher heat resistance. In addition, the star polymer with particular terminal group has also wide applications in adhesives, liquid rubber etc.
However, this initiator is not much used in practice, because of inaccessibility starting materials, complex manufacture technology, and functionality being neither easily controlled nor regulated, although which can be used in the synthesis of star block copolymer (such as EP 0,210,016, U.S. Pat. No. 4,161,494, U.S. Pat. No. 4,196,153, U.S. Pat. No. 172,190 etc.), and is rarely used in the synthesis of other star polymers. For example, in GB 2124228, the initiator thereof is prepared by the addition of a divinyl aromatic hydrocarbon and an alkyllithium.
The process that RLi is used as the initiator in an aromatic hydrocarbon solvent to initiate a small amount of divinylbenzene (DVB) to prepare multifunctional organic lithium has already been reported in literatures such as GB 2,124,228; and Helmut Eschwey, Walther Burchard, Polymer, vol. 16, 181, 1975. The functionality of the initiator is controlled by the sorts of RLi (n-BuLi, s-BuLi, t-BuLi), the position of the DVB substituent (p-DVB) and the feed ratio between RLi and DVB.
In the above-stated process, other compounds containing several double-linkages can also be substituted for DVB, such as multivinyl aromatic hydrocarbon compounds: 1,2,4- trivinyl benzene, 1,3,5-trivinylnaphthalene etc.; multivinylphosphor compounds: methyl divinyl phosphor, trivinylphosphor etc.; multivinylsilicon: methyl trivinylsilicon, diethyldivinylsilicon etc., for example, those disclosed in U.S. Pat. No. 4,091,198.
In order that the multifunctional organic alkali metal initiator thus prepared has a higher solubility in non-polar solvent, it was also reported that employing similar processes, e.g. using much larger molecular weight organic compound containing two or three double-linkages to replace DVB to prepare multifunctional organic lithium initiator in an aromatic hydrocarbon or an alkane as well as using RLi as the initial initiator, for example ##STR1## in U.S. Pat. No. 4,196,153; EP 210,016; and ##STR2## in U.S. Pat. No. 4,161,494; theoretically speaking, the functionality will continuously rise along with the reaction is carried out, but it is generally about 3 as reported.
The multifunctional organic lithium initiator can also be prepared through using RLi to initiate a small amount of a mixture containing a diene and DVB, for example, U.S. Pat. No. 4,091,198.
Further, the multifunctional organic lithium initiator can also be prepared by reacting RLi with the oligomer containing several double-linkages, such oligomer may be polydiene (D. D. U.S. Pat. No. 150,469; D. D. U.S. Pat. No. 158,781), and may also be the "microgelling core" having several double-linkages and formed by the copolymerization of styrene and DVB (Koji Ishizu, Shinichi Gamoo, Takashi Fukutomi, Toshio Kakurai, Polymer J., Vol. 12,399, 1980; Yin Rui-Liu Hao, and Shen Jiacong, The Academic Thesis Photoprint Collection of the National High Molecular Polymer Report Conference, 124, 1987).
On the other hand, the multifunctional organic alkali metal initiator can also be formed by using low molecular weight active polymer to replace RLi to react with DVB etc. (Rempp. P., Iranta. E, Pure Appl. Chem. 30, 229, 1972; Yin Rui, and Shen Jiacong, The Academic Thesis Photoprint Collection of the National High Molecular Polymer Reporting Conference, 126, 1987). The functionality of the initiator is principally influenced by the magnitude of the molecular weight of the active polymers used, the smaller the molecular weight, the higher the functionality, and the scope of the functionality is generally found to be 3.about.100.
The S-SBR has been put into industrial production by several corporations abroad, they employ basically the second process, such as BP 2,160,207, U.S. Pat. No. 4,397,994, U.S. Pat. No. 4,519,431, U.S. Pat. No. 4,540,749, U.S. Pat. No. 4,603,772, JP 82,205,414 etc.; the coupling agent is generally the metal halide containing several halogen atoms as well as divinyl benzene etc. If dilithium initiator is used, R.sub.2 S.sub.n X.sub.2 is generally used as a coupling agent so as to avoid gel formation, for example U.S. Pat. No. 4,742,174. In U.S. Pat. No. 4,742,174, there is introduced a solution copolymerization reaction which is carried out by using an organic lithium multifunctional initiator; and then a coupling reaction with the feed including R.sub.2 S.sub.n X.sub.2 is carried out to prepare a random copolymer of conjugated diene-vinyl substituted aromatic hydrocarbon; from which a composition containing at least 20% of the copolymer can be produced so as to provide a rubber composition useful for improving tire performances.
There exist the following problems to be solved in the art:
(1) the functionality can not be stably controlled nor regulated, for example, in case of RLi+DVB, it is difficult to be used in the industry synthesis of star polymers; PA1 (2) the synthesis of some starting materials themselves is complicated, their purity can not be guaranteed, which can influence the functionality of the final product, such as ##STR3## (3) The production cost of RLi is rather high; (4) There exist no other hetero-atoms in the initiator than carbon, hydrogen, and alkali metal, while with respect to star polymers, in the case that there is contained the hetero-atoms on the macromolecular chain such as the star S-SBR with its macromolecular chain containing the Sn-C linkage, there will appear better carbon black absorption and dispersion for processing the same, which is favorable for improving the strength.
Just because of the above-stated insufficiency of the prior art, the existing star polymer synthesis techniques rarely used multifunctional organic lithium but RLi generally as the initiator, i.e., firstly synthesizing active linear high polymer, then adding thereinto a coupling agent to carry out a coupling reaction, and finally forming the star polymer. These exist many limitations on such synthesis process, i.e., complex technology, low utility rate of the equipments, big investment, acute requirements on the equipments and the polymerization conditions, large energy consumption, low coupling rate, and also unsatisfactory product performances.
In order to find the method for solving the above-indicated problems, the present inventors conducted a deep going research and many tests thereon, so finally there comes into appearance a breakthrough and the present invention is accomplished.